fix(security): no direct HTTP access to plugin PHP files (audit L2, cheap part)

Every file under src/ exits outside WordPress (defined('ABSPATH') ||
exit; after the use block, so a direct request no longer prints a fatal
error with the server path); divi-convert-fixture.php gets the WP_CLI
guard the other integration scripts have. A unit test keeps both true;
PSR1 side-effects warning excluded for the guard.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

tests/Integration/divi-convert-fixture.php

@@ -7,6 +7,8 @@
  * Paths are relative to the plugin's tests/Fixtures/divi directory.
  */
 
+defined('WP_CLI') && WP_CLI || exit;
+
 use ET\Builder\Packages\Conversion\Conversion;
 
 $args = $args ?? [];